The present invention relates generally to generally to trace gas detection and more specifically to cavity enhanced absorption spectroscopy systems and methods for measuring the trace gases.
In optical absorption spectroscopy systems and methods, optical intensity inside the resonance cavity reflects total cavity loss at the moment when the laser light frequency coincides with a cavity mode transmission peak. The total cavity loss is a sum of the cavity mirror losses and losses caused by absorption of a gas mixture present in the cavity. However, the intra-cavity optical power depends also on the coupling efficiency of the laser beam to the particular cavity mode. In practice, it is difficult to precisely estimate the coupling efficiency as a lot of parameters affect it, such as spatial, polarization, and spectral overlapping of laser and cavity modes. Moreover, this efficiency can vary over time causing a drift.
In traditional cavity enhanced absorption spectroscopy methods, the intensity of the light transmitted by the cavity is normalized on the intensity of the light incident on to the cavity. In these approaches, all fast non-correlated fluctuations of longitude and transverse intensity modes of the laser beam cause an additional, unwanted noise in the absorption measurements. Also, slow changes of the laser and cavity mode overlapping cause an undesirable drift of the base line.
Therefore it is desirable to provide systems and methods that overcome the above and other problems.